CVE-2021-43566

All versions of Samba prior to 4.13.16 are vulnerable to a malicious client using an SMB1 or NFS race to allow a directory to be created in an area of the server file system not exported under the share definition. Note that SMB1 has to be enabled, or the share also available via NFS in order for this attack to succeed.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Do not enable SMB1 (please note SMB1 is disabled by default in Samba from version 4.11.0 and onwards). This prevents the creation of symbolic links via SMB1. If SMB1 must be enabled for backwards compatibility then add the parameter:

unix extensions = no

to the [global] section of your smb.conf and restart smbd. This prevents SMB1 clients from creating symlinks on the exported file system.

However, if the same region of the file system is also exported using NFS, NFS clients can create symlinks that potentially can also hit the race condition. For non-patched versions of Samba we recommend only exporting areas of the file system by either SMB2 or NFS, not both.